1. Field of Invention
The present invention relates generally to the field of servers and pertains more particularly to a system for providing reliable client/server sessions by controlling the admission of arriving messages to a server.
2. Discussion of the Prior Art
Servers are commonly employed for sharing of information among large numbers of computer systems or similar devices. A computer system or similar device that communicates with a server is usually referred to as a client of the server and the server is often part of a host system. A client and a host typically exchange messages via a communication network using a predetermined protocol. Such protocols are usually arranged in a client/host model in which a requesting client transfers a request message to a host and the host in turn takes an appropriate action depending on the content of the request message. Typically, the appropriate action for the request message includes the transfer of a response message to the requesting client.
Prior protocols typically do not allow for the establishment of a persistent session between the client and the host in the traditional sense in which a local terminal establishes a session on the computer system. Instead, any session-like information is usually implied in the content of the messages exchanged between the client and the host. Such a communication protocol may be referred to as a xe2x80x9cstatelessxe2x80x9d protocol. Such stateless protocols include protocols associated with Internet communication including the Internet Protocol (IP), the User Datagram Protocol (UDP), the Simple Mail Transfer Protocol (SMTP), and the Hypertext Transfer Protocol (HTTP), as well as the Network File System (NFS) Protocol.
A client that accesses a host commonly engages in an extended transaction with the host. Such an extended transaction typically involves the exchange of multiple messages between the client and the host. For example, an NFS client typically issues multiple request messages to an NFS server while retrieving a file from the NFS server. Similarly, an HTTP client typically issues multiple request messages to an HTTP server while browsing through web pages contained on the HTTP server. Such transactions that involve the exchange of multiple messages between a client and a server are hereinafter referred to as sessions.
Servers commonly have a large pool of potential clients which may issue request messages. For example, an HTTP server connected to the world-wide-web has potentially millions of clients from which it may receive request messages. Prior servers that are adapted for stateless protocols typically respond to each request message in the order in which it is received, that is, on a first-come-first-served basis regardless of the source of the request message.
In the present context, the term xe2x80x9cquality of servicexe2x80x9d refers both a host""s ability to provide quick response to a message and to complete an entire session. As a particular host becomes more popular, and due to that popularity receives more messages, the host""s processing resources can become stretched. For example, due to heavy traffic, a host may not be able to respond to a message at all, or the host may not provide a timely response which can cause a client to xe2x80x9ctime-outxe2x80x9d and generate an error. Poor quality of service can have significant results, as users may become frustrated and simply give up trying to reach a particular host, or the sponsor of the host may lose sales or fail to communicate needed information to any or all clients.
Two techniques are generally used to alleviate quality of service problems. First, more processing capacity can be added to the host, typically by either replacing the host with another, more powerful computer, or by providing multiple computers in parallel and delegating new messages to different ones of the multiple computers. While this first technique presents an effective way of reducing some quality of service problems, it is not always practical. For example, sometimes, due to inadequate planning, budgetary constraints or space constraints, additional processing capacity simply cannot be added. Other times, if demand for a host is not properly forecast, there may be a long lead time before additional processing capacity can be purchased and implemented.
A second technique calls for applying xe2x80x9cadmission control,xe2x80x9d where only a certain set number of client messages are processed (xe2x80x9cadmittedxe2x80x9d) and the remainder are refused. Of the messages which are in fact admitted, all are ideally handled in an expedient manner without degradation of quality of service as to those admitted messages. An advantage of this technique is that admission control can be implemented in software, thus facilitating quick, inexpensive use with little advance notice. Unfortunately, typical admission control mechanisms operate by admitting messages on a message-by-message basis, and so, these typical admission control techniques do not provide an adequate solution for multiple-message sessions. Also, the messages which are not admitted to the host are generally not handled at all, such that a client is not informed that the request has been refused or the client, if informed, is simply asked to xe2x80x9ctry again later.xe2x80x9d Typically, a refused client must try repeatedly to obtain service with no guarantee that future requests will be processed. For these reasons and others, techniques generally used to alleviate quality of service problems are not always successful.
A definite need exists for an admission control system having an improved ability to alleviate quality of service problems. In particular, a need exists for an admission control system which responds to all messages, whether or not those messages are actually admitted. Ideally, such system would operate by admitting entire sessions, not just individual messages, such that messages relating to a session in-progress are generally admitted. With a system of this type, admission control would at least provide a reliable means of finishing each session with high quality of service. Finally, a need exists for a system that provides some level of service to all clients, including those which have been refused admission. The present invention solves these needs and provides further, related advantages.
An admission control system for a server is disclosed including an admission controller that receives a stream of messages from one or more clients targeted for the server. The admission controller relays to the server the messages in the stream that correspond to a number of sessions already underway between the clients and the server. The admission controller also relays to the server the messages in the stream that do not correspond to sessions already underway if a hybrid and predictive admission control strategy using information provided by a resource monitor indicates that additional sessions can be handled by the server. The admission controller defers the messages otherwise.